Various types of pumps are used by medical personnel to infuse drugs into a patient's body. Of these, cassette infusion pumps are often preferred because they provide a more accurately controlled rate and volume of drug infusion than other types of infusion pumps. A cassette pump employs a disposable plastic cassette coupled in a fluid line extending between a drug reservoir and the patient's body.
In one prior art design of a cassette infusion pump, the cassette comprises a plastic shell or housing having a front section joined to a back section. A thin elastomeric sheet or membrane is encapsulated between the two sections. Fluid flows from one of two selectable inlet ports into a pumping chamber defined by a concave depression in one of the sections through passages formed in the housing. The cassette is inserted into an appropriate receptacle of a pump chassis that includes a microprocessor controller and a motor actuated driver. A plunger actuated by the motor in the pump driver displaces the elastomeric membrane to force fluid from the pumping chamber toward an outlet port under pressure. The pump chassis thus provides the driving force that pumps fluid through the cassette. The microprocessor control is programmable to deliver a selected volume of fluid to the patient at a selected rate of flow. In addition, the pump chassis may include one or more pressure sensors and air bubble sensors used to monitor the drug infusion process to protect against potential problems that may arise during the drug delivery.
Both single and multi-channel cassette pumps are available. A multi-channel cassette pump allows more than one type of medicinal fluid to be selectively delivered to a patient using a single pump cassette. Such pumps are frequently used in association with intravenous (IV) drug delivery therapies.
When the pump inlet and outlet pressure conditions are approximately equal, cassette type infusion pumps are quite accurate. However, when the pressures at the pump inlet and outlet vary substantially, the delivery accuracy of cassette pumps degrade. If the delivery rate is relatively low, as is often the case in pediatric applications, and if the differential pressure exceeds 3 psi, accuracy is significantly impaired, and retrograde flow can occur. In retrograde flow, fluid moves from the patient's vascular system towards the pump, which can result in blood from a patient being drawn out of the patient's body and into the IV line. Even if such retrograde flow occurs only briefly, and the accuracy of the delivery rate is not severely impaired, the visual impact of even a small amount of blood in an IV line can be extremely disturbing to care providers, patients, and visitors. Retrograde flow is more likely to occur if the pump fluid source is lower in elevation than the entry site of an IV line into the patient's body, because the inlet pressure is then lower than the outlet pressure due to the head pressure.
The effect that a differential pressure has on the accuracy of the flow rate of a cassette pump depends on whether the pressure at the pump inlet is higher or lower than the pressure at the pump outlet. A higher pump inlet pressure, which is typically due to an increased elevation of the fluid reservoir relative to the pump (i.e., the reservoir head pressure), often causes the flow rate to exceed the desired setting, which the pump is programmed to deliver. Conversely, a higher pump outlet pressure, which can be caused by a partially restricted fluid line connected to the pump outlet or by the entry site into the patient being disposed higher than the pump inlet, can cause the flow rate to decrease below the desired value.
In a balanced pressure environment, cassette pumps tend to act like constant displacement pumps, so that each pumping cycle delivers the same volume of fluid. The delivery rate of the fluid is controlled by varying the number of pumping cycles per unit of time; thus, higher delivery rates require more pumping cycles to be executed during a given time interval than lower delivery rates. The pumping cycle of the prior art cassette pump briefly described above corresponds to a plunger deflecting the elastomeric membrane into the chamber in which the constant volume of fluid is contained, thereby forcing the fluid from the chamber through an outlet valve. The position of the plunger is controlled by a microprocessor. It is possible to change the delivery pressure of the constant volume of fluid to be delivered into the fluid line that is coupled to the patient's body by adjusting the position of the plunger at the beginning of each pumping cycle. Because the fluid volume delivered during each cycle (and hence the volume of the chamber in which the fluid is contained) is relatively small (generally about 333 μl of fluid is delivered per cycle), a very small change in the initial plunger position will have a significant impact on the pumping chamber pressure.
Clearly, it would be desirable to provide a cassette pump in which a pressure compensated pumping cycle is used to minimize the effect of differential pressures between the inlet and outlet of the pump. A cassette pump achieving this benefit and having accurate flow rates under varying pressure conditions is not disclosed in the prior art. Preferably, such a system would use a multi-component pressure-kinetic model to determine the pressure compensation required due to a differential pressure between the inlet and outlet of the cassette pump. Such a system would preferably use real-time measurements of pressure at both the pump inlet and pump outlet to determine the differential pressure, and then use an empirically determined algorithm to determine the extent to which the position of the plunger should be adjusted to either increase or decrease the delivery pressure. The delivery rate can further be optimized by changing the rate of the pumping cycles as a function of the actual volume delivered during each pump cycle. Preferably such a model would be used to pressure compensate the delivery of medicinal fluids for single or multi-channel cassette pumps. It will thus be apparent that accurately controlling the administration of medicinal fluids under varying pressure conditions using a pressure compensation model would provide significant advantages over the prior art.